1. Field of the Invention
The instant disclosure relates to an operational amplifier, and more particularly, to an operational amplifier that is capable of minimizing impact of an offset voltage by adjusting a gain thereof through manipulation of a current mirror ratio of a current mirror and a load.
2. Description of Related Art
Operational amplifiers have been widely utilized in numerous electronic components such as buffers, filters, and digital-to-analog converters.
Ideal operational amplifiers are associated with characteristics including infinite input resistance, zero output impedance, infinite open loop gain, infinite common mode rejection ratio, and indefinite bandwidth. However, due to limitations to the manufacturing process of the operational amplifiers it is almost impossible for the real-world operational amplifiers to possess the above-mentioned characteristics. And the deviation from the ideality is traditionally represented in terms of an offset voltage and the presence of the offset voltage needs additional mechanism of compensation.
For compensating the offset voltage, a traditional operational amplifier device having a unit gain operational amplifier, an offset voltage compensating unit and an equivalent voltage source is provided. The unit gain operational amplifier is associated with a gain of “1” for equaling an output voltage to an input voltage. The unit gain operational amplifier further includes an input circuitry and an output circuitry, while the equivalent power source is for representing the deviation from the ideality of the operational amplifier device and is not actually present in the operational amplifier. Specifically, an output voltage of the equivalent power source corresponds to the offset voltage of the unit gain operational amplifier, and the offset voltage compensating unit, which includes at least three switches and a capacitor, is for compensating the offset voltage taking advantage of the characteristic of the “virtual shorting” of the unit gain operational amplifier.
When traditional the operational amplifier device operates in an offset voltage storage mode, two of three switches are conducted and the remaining one switch is turned off, causing the voltage difference between a positive input terminal and an output terminal of the unit gain operational amplifier, which is the so-called offset voltage, to charge the capacitor. And when the traditional operational amplifier operates in an offset voltage compensating mode the switch that is closed when the offset voltage storage mode is entered becomes conducted, with the switches that are turned on when the offset voltage storage mode is entered being turned off. As such, the voltage difference that is stored in the capacitor over the course of the offset voltage storage mode could help minimize the impact of the offset voltage, reducing the voltage difference between the positive input terminal and the output terminal of the unit gain operational amplifier to be approaching to zero.
When the output voltage of the operational amplifier device varies along with changes in the input voltage, the offset voltage compensating unit operates to compensate the impact of the offset voltage by switching on/off the switches.
However, when the output terminal of the unit gain operational amplifier is coupled to another capacitor, changes in the output voltage in accordance with the changes in the input voltage are affected by a slew rate of the unit gain operational amplifier. Thus, the changes in the output voltage could not synchronize with the changes in the input voltage, resulting in discrepancy between the input voltage and the output voltage at least at certain points of time. Consequently, the voltage stored in the capacitor may not be equal to the offset voltage and cannot be entirely compensated when the traditional operational amplifier device operates in the offset voltage compensating mode, undermining the performance of the traditional operational amplifier device.